Together with He Weifang, another prominent Chinese lawyer, Mr Mo was questioned at the airport for 40 minutes by the police and told that his trip to the UK posed “a threat to China’s national security”.

Mr Mo, who argued in court that Mr Liu’s conviction was against China’s constitution, has so far been able to avoid the detentions and house arrest orders that have been meted out to scores of Mr Liu’s other supporters. By blocking him at the airport, China breached article 13 of the United Nations Declaration of Universal Human Rights, to which it is a signatory. The article states: “Everyone has the right to leave any country, including his own, and to return to his country.”

The lawyer put more pressure on Mr Cameron to speak up against China’s abuses, saying: “No country should avoid talking about human rights with China in order to boost their economic interests, and I am glad the UK has chosen not to go down that route.”

Mo Shaoping was invited, along with more than 100 others, by Liu Xiaobo’s wife to attend the Nobel award ceremony in her husband’s stead. Mo earlier told reporters he believed he would be prevented from leaving the country to attend the ceremony. However, during his recent run-in at the airport, Mo told the AFP that he was planning to attend an international legal conference in Britain and was not planning to travel to Oslo.

Editor’s note: Mo Shaoping did not in fact represent Liu Xiaobo in court, as he was barred from doing so because he was a signatory of Charter 08, the pro-human rights and democracy manifesto drafted by Liu Xiaobo. Read more in this essay by Mo Shaoping about Liu Xiaobo’s case. (h/t @siweiluozi)